Surface finishing apparatus and method



March 1970 P. F. FORMAN ET AL 3,503,781

SURFACE FINISHING APPARATUS AND METHOD I 2 Sheets-Sheet 1 Filed Dec. 29, 1965 Q 'gj INVENTORS. Paul 1 firmcuz BY Edward. LBoaaca March 31, 1970 P. F.,FORMAN ET AL SURFACE FINISHING APPARATUS AND METHOD 2' Sheets-Sheet 2 Filed Dec. 29, 1965 INVENTORS'. Paul I. Forman BY Edward L. Balzac United States Patent US. Cl. 117 -38 7 Claims ABSTRACT OF THE DISCLOSURE A technique and apparatus for finishing a surface to a desired contour by depositing on the surface a single layer of material whose thickness is non-uniform and controlled. First, an interferogram is made of the surface to determine its existing contour. The interferogram is then compared with an interferogram of the surface as it should appear when finished so as to determine the location, shape and depth of any errors. Using this information, a mask is prepared whose over-all size and shape correspond to the size and shape of the surface and which contains a pinrality of slots individually sized so that the width of any slot at any point through its length is directly proportional to the error in depth at the corresponding location on the surface. Material is then deposited by evaporation through the mask onto the surface.

This invention relates to a technique for finishing a surface. More particularly, this invention relates to a technique for figuring or correcting a surface to a desired contour by depositing on the surface a coating of material. The invention is especially useful in finishing planar, spherical, aspherical and other shaped surfaces of optical elements but may also be applied to figuring surfaces of non-optical elements.

Essentially, the invention involves a process wherein the thickness of a coating that is deposited on different locations on a surface is made dependent, at least partially, on the topography of the surface.

It is well known to correct or figure optical surfaces by polishing techniques. Basically, however, polishing is a subtractive process in which defects in a surface are corrected by removing high spots. Furthermore, polishing is an averaging type process which usually takes place over relatively large areas of the surface. Localized defects are corrected by changing the various polishing parameters, that is, speed, stroke and pressure to favor the defects. In any event, even though the defective area may be very small in size, if it is a low area the polisher cannot work directly on the area but must polish the entire surface while relying on his skill and experience in correcting the above-mentioned parameters. Other techniques, such as for example, replication and grinding have also been employed in finishing optical surfaces.

It has been proposed to make use of interference fringes incorrecting errors in optical surfaces. However, the proposed use for the fringes has been limited to providing a visual aid in showing the polisher the approximate location of the high (or low) spots.

It has also been proposed to prepare certain types of optical surfaces such as parabolas by evaporating an optical material on the surface through a rotating mask. However, because of the rotation of the mask this technique is only useful in depositing a coating of symmetrically variable thickness and does not take into consideration the actual topography of the surface before the coating is applied.

Accordingly, the need has arisen for a new technique for correcting or figuring a surface.

It is, therefore, an object of the present invention to provide a new method and apparatus for figuring a surface.

It is another object of the present invention to provide a new and improved method and apparatus for correcting or figuring an optical flat.

It is still another object of the present invention to provide a new and improved method and apparatus for correcting an optical surface by depositing a coating of material onto the surface.

It is yet still another object of the present invention to provide a new and improved method for making a mask which is representative of the contour of a surface.

It is another object of the present invention to provide a means for correcting by evaporative deposition nonsymmetrical deflects in an optical surface.

It is still another object of the present invention to provide a new and improved method and apparatus for correcting an optical surface by additive means.

It is yet still another object of the present invention to provide a new method and apparatus for correcting an optical surface by depositing a material onto the surface which utilizes interference fringes to determine the thickness of coating required.

It is another object of the present invention to provide a new method and apparatus for correcting an optical surface to of a wavelength or better.

It is still another object of the present invention to provide a new and improved mask for use in depositing an evaporant in an uneven predetermined manner onto a substrate and a new and improved process of fabricating said mask.

It is yet still another object of the present invention to provide a new and improved compensator for use in an evaporative coating apparatus.

It is another object of the present invention to provide a new and novel type vacuum coating apparatus.

The above and other objects are achieved according to this invention in which errors in the contour of a surface are corrected by depositing on the surface a coating of material whose thickness at any location is dependent on the error in the contour at that location. The coating is deposited on the surface using vacuum deposition type apparatus. The apparatus includes a novel compensator and a novel specially prepared mask. The compensator is designed to provide a uniform distribution of the evaporant (coating material) emerging from the source. The mask is constructed so that the amount of material deposited through it onto the surface is dependent on the existing contour of the surface and the desired contour of the surface. The contour of the surface is determined by taking an interferogram of the surface and the mask is made utilizing the interference fringes so produced.

A more complete appreciation of the invention and other attendant advantages thereof will be readily appreciated as the same becomes better understood through reference to the following detailed description when taken in conjunction with the accompanying drawings wherein like reference numerals represent like parts and wherein:

FIGURE 1 is a plan view of an optical element,

FIGURE 2 is a section view of the optical element shown in FIGURE 1, taken along an axis 22 and exaggerated somewhat to show more clearly the contour of its upper surface,

FIGURE 3 is a section view of optical element shown in FIGURE 1 taken along an axis 33 and exaggerated somewhat as in FIGURE 2,

FIGURE 4 is a diagrammatic chart made up of an interferogram of the upper surface of the element shown in FIGURE 1 and an interferogram of a plane surface,

FIGURE 5 is a plan view of a mask prepared according to this invention,

FIGURE -6 is a plan view of a compensator constructed according to the invention,

FIGURE 7 is an enlarged view of one of the slots in the mask in FIGURE 5, and

FIGURE 8 is a simplified plan view partly in section of a coating apparatus constructed according to the invention including the mask shown in FIGURE and the compensator shown in FIGURE 6.

Referring now to the drawings, and in particular FIG- URES 1 through 3, there is shown an optical element 11 having an unfinished top surface 12. As can be seen, surface 12 is generally flat except for two low spots. In the FIGURE 1 view, one of these low spots is at the upper left and the other low spot is at the lower right.

Assume that for a particular application, surface 12, when properly finished, should be flat.

According to this invention, the technique for finishing a surface is by adding material. Obviously, more material must be deposited on the low spots than on the other portions of the surface if the low spots are to be eliminated. In order to determine the amount of material (thickness) that should be deposited over different locations on the surface it is necessary to accurately determine the existing contour of the surface.

One way of determining the exact contour of surface 12 is by means of an interferogram. Any of the well known types of interferometers, such as a multiple pass interferometer may be used in making the interferogram.

As is well known in the art, the interference fringes produced by making an interogram of a surface are substantially equal to the contour lines of the surface. The shape and spacing of these lines is a direct representation of the contour of the surface.

Referring now to FIGURE 4, there is shown a chart 21 which includes an interferogram of surface '12 before it is finished. The lines 22-28 represent the interference fringes of the surface. Also shown on the chart 21 are a plurality of reference lines 32-38. These reference lines 32-38 are spaced in substantially the same way as the interference lines 22-28 of the surface 12 and represent an interferogram of how surface 12 should look when finished flat. As can be seen the interference fringes 32-38 are substantially equally spaced, parallel straight lines.

By comparing the interference lines 22-28 with the reference interference lines 32-38, one can readily see where surface 12 deviates from the desired flat shape and the relative magnitude of the errors. Using this information a novel type partially transmissive mask 41 such as shown in FIGURE 5 is then prepared. The mask 41 comprises a thin sheet of material such as copper which is etched, perforated or otherwise cut with a plurality of closely spaced substantially parallel lines or slits 42, 43, 44, 45, etc. The shape of mask 41 is the same as the shape of surface 12. The size of mask 41 is proportional to the size of surface 12. In the drawings, the size of mask 41 is about equal to the size of surface 12. The width throughout the length of each slot 42-45 etc. is variable and is determined by examining the two groups of lines on the interference chart shown in FIGURE 4. The examination involves measuring the point to point distance between an interference line in one of the groups and its companion interference line in the other group. The mask 41 is further provided with a plurality of parallel equally spaced supporting struts 51. The purpose of struts 51 is to prevent any distortion in the size or shape of the slots 42-45 etc. during the coating operation due to the narrow width of the non-transmissive portions of the mask. The struts 51 are positioned at an angle to the slots 42-45 etc. so as to cause a uniform attenuation across the surface being coated.

Referring now back to FIGURE 4, there is shown a plurality of closely spaced parallel lines 62-65 etc. intersecting the two groups of interference fringes. Each one of the lines 62-65 etc. corresponds to the longitudinal 4 axis of one of the slots 42-45 etc. in the mask 41. The width of each slot may change over its length is determined by examining each pair of interference fringes it intersects. The distance between the two interference fringes determines the particular thickness at that point of intersection.

The size and shape of the slots can be more easily seen by looking at FIGURE 7 which is a greatly enlarged view of the slot 45 shown in the mask in FIGURE 5. As can be seen in FIGURE 7, the slot 45 is substantially wider at the left portion. By looking at the chart in FIG- URE 4, it can be seen that as line 65 is examined from one end to the other the distance between the actual interference line and the reference interference line decreases from left to right.

It is to be understood that this examining procedure could also be done automatically by an electromechanical system using an electronic computer to scan the interferogram and then fabricate the mask.

Referring now to FIGURE 8, there is shown an apparatus for deposting a coating of material onto the surface 12 of the optical element 11. The apparatus includes an evaporating tank 71 for depositing by evaporation a suitable material, such as silicone oxide, onto the sur' face of the element 11. The evaporating tank 71 is essentially of the type well known in the art and includes a bell jar or housing 72, a source of evaporant 73, a power supply 74 for activating the evaporant, and a bracket assembly 75 on which is mounted the optical element 11.

In order to provide for a controlled rate of flow of the evaporant from the source 73, a thickness monitor (not shown) of the type well known in the art is connected to the source of evaporant.

In order to provide for a radially uniform distribution of coating material from the source 75, a compensator 76 is mounted within the tank 71 for rotatable movement and positioned between the optical element 11 and the source of evaporant 73. The shape of the compensator 76 can best be seen by referring to FIGURE 6. The compensator is mounted on a shaft 77 which is mechanically coupled by a magnetic clutch 78 to a motor drive 79. The compensator 76 is carried by support 80.

The partially transmissive mask 41 is positioned between the compensator 76 and the optical member 11 in its proper orientation.

In order to deposit the material over the surface 12 in a smooth manner and eliminate voids at the nontransmissive portions of the mask 41, means are also provided for oscillating the mask 41 linearly and bi-directionally in a direction perpendicular to the mask slots 42-45 etc. These means include a drive motor 81, a constant velocity cam 82, a spring 83 and suitable mechanical linkage 84. The mask 41 is oscillated an integral number of line cycles. The term line cycle as used herein is intended to refer to the distance from a point on one side of one slot to a corresponding point of same said side of an adjacent slot. The size of these line cycles is equal throughout the entire mask 41.

It should be noted that the above-described process for making the mask 41 and then coating the surface 12 is iterative and can be repeated a plurality, of times.

The invention has actually and successfully been used to figure a surface. In one instance an optical surface was figured to an accuracy of better than of a wavelength.

In another embodiment of the present invention, a mask is prepared directly from the interferogram without scanning or otherwise examining the interferogram as described above. In this embodiment the size and shape of the slots are determined directly from the two groupings of itnerference lines and are formed simply by cutting out the area between each companion pair of interference lines.

The mask is then mounted within the bell jar in a manner similar to that described above and during the coating operation is oscillated linearly and bi-directionally along an axis substantially perpendicular to the reference interference lines.

Obviously many modifications and variation of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced other than described above.

What is claimed is:

1; A method of finishing a surface of an optical element to a desired contour by adding a layer of material of predetermined variable thickness to the surface, the method comprising the steps of:

(a) determining the locations of the errors in depth in the contour of the surface,

(b) preparing a mask made of a sheet of material having a plurality of elongated parallel slots of nonuniform width, the slots covering an area on the sheet of material corresponding in shape to the shape of the surface, the relative width of any slot at any point throughout its length being directly proportional to the error in depth at the corresponding point on the surface,

(c) placing the mask in front of and in geometric registration with the surface,

(d) directing a beam of material of uniform cross-sectional density onto the mask, and

(e) oscillating the mask at right angles to the longitudinal axes of the slots,

whereby the relative amount of material deposited at any location on the surface through the mask will be directly proportional to the error in depth at that location on the surface.

2. The method of claim 1 and wherein determining the locations of the errors in depth in the contour of the surface comprises the steps of:

(a) making an interferogram of the surface of the optical element,

(b) making an interferogram of the surface as it should appear when corrected to the desired contour, and

(c) comparing the two interferograms to determine the locations and magnitudes of the errors.

3. A method of making a mask adapted to receive a beam of coating material of uniform cross-sectional density and transmit onto a surface a beam of coating material whose cross-sectional density varies so as to be directly proportional to the errors in depth in the contour of the surface, comprising the steps of:

(a) providing a sheet of nontransmissive material having an area corresponding in shape to the area of the surface,

(b) determining the locations of the errors in depth on the surface, and

(c) perforating the area on the sheet of material so as to form a plurality of elongated parallel slots of nonuniform width, the relative width of any slot at any point throughout its length being directly proportional to the error in depth at the corresponding point on the surface.

4. A mask for use in regulating the amount of material deposited onto different locations on an optical surface so as to correct errors in the contour of the surface comprising:

a sheet of material having an area corresponding in shape to the shape of the surface, said area comprising transmissive and non-transmissive portions,

the relative size of the transmissive portions at any location corresponding to the error in depth in the contour at the corresponding location on the surface.

5. The mask according to claim 4 and wherein said transmissive portions are in the form of a plurality of parallel slots, the relative width of each slot relative to the other slots and the relative width of each slot throughout its length being related to the errors in the contour of the surface.

6. Apparatus for use in finishing a surface of an optical element to a desired contour comprising:

(a) a vacuum chamber,

(b) first means located near the bottom of the chamber for producing an upwardly directed beam of evaporant material of uniform cross-sectional density,

(c) second means located within the chamber above the first means for supporting an optical element whose surface is to be finished,

(d) a mask located in the chamber between the first means and the second means for altering the crosssectional density of the beam of material impinging on the surface, the mask comprising a sheet of material having an area corresponding in shape to the area of the surface, the area on the mask having a plurality of elongated parallel slots of nonuniform width, the relative width of each slot at any point throughout its length being directly proportional to the error in depth at the corresponding point on the surface, and

(e) means for oscillating the mask at right angles to the longitudinal axes of the slots.

7. The apparatus according to claim 6 and wherein the first means includes an evaporant source for producing a diverging beam of evaporant material and a compensator for correcting the nonuniform cross-sectional density of the beam of evaporant emerging from the evaporant source.

References Cited UNITED STATES PATENTS 2,408,614 10/ 1946 Dimmick 118-49 2,432,950 12/1947 Turner et al. 118-49 2,549,926 4/1951 Pride 'et al 117-38 X 2,771,055 11/1956 Kelly et a1 117-106X 3,117,025 1/1964 Learn et al. 118-49 3,148,085 9/1964 Wiegmann 117-38 X 3,228,794 1/1966 Ames 117-38 X 3,230,109 1/ 1966 Domaleski 117-38 X 2,954,301 9/ 1960 Szukiewicz 264-36 X FOREIGN PATENTS 569,046 5/ 1945 Great Britain.

OTHER REFERENCES Strong and Gaviola: On the Figuring and Correcting Mirrors by Controlled Deposition of Aluminum, J.O.S.A. vol. 26, April 1936, pp. 153-162.

ALFRED L. LEAVITT, Primary Examiner A. GRIMALDI, Assistant Examiner US. Cl. X.R. 

